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| United States Patent |
5,523,451
|
|
Rechner
, et al.
|
June 4, 1996
|
Process for the continuous preparation of aryl carbonates
Abstract
Organic carbonates which contain at least one aromatic ester group can be
obtained continuously from carbonates, which contain at least one
aliphatic ester group, and a phenolic compound in the presence of a
transesterification catalyst known per se in that the reaction is carried
out in a bubble column reactor or in a cascade of at least two bubble
column reactors in such a way that the phenolic compound is metered into
the first bubble column and the carbonate containing at least one
aliphatic ester group is metered into each individual bubble column, but
preferably only into the last bubble column. The carbonate containing at
least one aromatic ester group is taken off in the liquid state from the
last bubble column. Volatile reaction products, for example eliminated
alcohol or a dialkyl carbonate are taken off at the upper end of each
individual bubble column, preferably at the upper end of the first bubble
column.
| Inventors:
|
Rechner; Johann (Krefeld, DE);
Schon; Norbert (Krefeld, DE);
Wagner; Paul (Dusseldorf, DE);
Buysch; Hans-Josef (Krefeld, DE);
Kabelac; Stephan (Langenfeld, DE)
|
| Assignee:
|
Bayer Aktiengesellschaft (Leverkusen, DE)
|
| Appl. No.:
|
206575 |
| Filed:
|
March 4, 1994 |
Foreign Application Priority Data
| Mar 12, 1993[DE] | 43 07 852.4 |
| May 17, 1993[DE] | 43 16 428.5 |
| Current U.S. Class: |
558/270; 558/274 |
| Intern'l Class: |
C07C 069/96 |
| Field of Search: |
558/274,270
|
References Cited
U.S. Patent Documents
| 4410464 | Oct., 1983 | Hallgren | 558/270.
|
| 5210268 | May., 1993 | Fukuoka et al. | 558/270.
|
| 5362901 | Nov., 1994 | Wagner et al. | 558/270.
|
| Foreign Patent Documents |
| 0461274 | Dec., 1991 | EP.
| |
| 3308921 | Sep., 1983 | DE.
| |
| WO92/18458 | Oct., 1992 | WO.
| |
Other References
Patent Abstract of Japan, JP 4230242, Aug. 19, 1992 vol. 16, No. 581
(C-1012); Fukuoka Shinsuke, "Continuous Production of Diaryl Carbonate";
p. 1.
Patent Abstracts of Japan, JP 4235951, Aug. 25, 1992, vol. 16, No. 584
(C-1013) ; Fukuoka Shinsuke, "Continuous Production of Diaryl Carbonate";
p. 1.
|
Primary Examiner: Dees; Jose G.
Assistant Examiner: Jones; Dwayne C.
Attorney, Agent or Firm: Gerstenzang; William C.
Sprung Horn Kramer & Woods
Claims
What is claimed is:
1. A process for the preparation of an aromatic carbonate of the formula
R.sup.1 --O--CO--O--R.sup.2 (I)
in which
R.sup.2 denotes phenyl or naphthyl each of which may be monosubstituted to
trisubstituted by straight-chain or branched C.sub.1 -C.sub.4 -alkyl,
straight-chain or branched C.sub.1 -C.sub.4 -alkoxy, cyano and/or halogen,
and
R.sup.1 independently of R.sup.2, assumes the range of meanings of R.sup.2
or denotes straight-chain or branched C.sub.1 -C.sub.6 -alkyl,
by catalyzed reaction of in each case 0.1-10 mol of an organic carbonate
having at least one aliphaltic ester group of the formula
R.sup.1 --OCOO--R.sup.3 (II)
in which
R.sup.3 denotes straight-chain or branched C.sub.1 -C.sub.6 -alkyl and
R.sup.1 has the above range of meanings,
with in each case 1 mol of a phenolic compound of the formula
R.sup.2 --OX (III)
in which
R.sup.2 has the above range of meanings and
X represents hydrogen or --CO--O--C.sub.1 -C.sub.6 -alkyl having a
straight-chain or branched alkyl group,
in the presence of a transesterification catalyst at
80.degree.-350.degree. C. and 10 mbar to 20 bar, wherein the reaction is
carried out in a bubble column reactor or a cascade of at least two bubble
columns in such a way that the phenolic compound of the formula (III) is
metered in liquid form into the first bubble column and the organic
carbonate of the formula (II) is metered in the liquid or gaseous state
simultaneously into each individual bubble column, in the case of liquid
metering, an evaporation of (II) in the bubble column proceeding, and the
reaction products of the formula (I) are taken off from the last bubble
column in liquid form and simultaneously at the upper end of each
individual bubble column the products of the formula
R.sup.3 --OX (IV)
in which R.sup.3 and X have the meaning mentioned, are taken off in gaseous
form.
2. The process of claim 1, wherein a dialkyl carbonate of the formula
R.sup.3 --O--CO--O--R.sup.3 (VI)
in which
R.sup.3 denotes straight-chain or branched C.sub.1 -C.sub.6 -alkyl is
reacted as the organic carbonate.
3. The process of claim 1, wherein a phenolic compound of the formula
R.sup.12 --OH (V)
in which
R.sup.12 denotes phenyl or phenyl monosubstituted by C.sub.1 -C.sub.4
-alkyl, C.sub.1 -C.sub.4 -alkoxy or chlorine
is reacted as the phenolic compound.
4. The process of claim 1, wherein 0.2-5 mol of organic carbonate is
reacted with 1 mol of phenolic compound.
5. The process of claim 4, wherein 0.5-3 mol of organic carbonate is
reacted with 1 mol of phenolic compound.
6. The process of claim 1, wherein, in the case of a cascade, the organic
carbonate is metered in only into the last bubble column.
7. The process of claim 1, wherein the products of the formula (IV) are
taken off in gaseous form, in the case of a cascade, at the upper end of
the first bubble column.
8. The process of claim 1, wherein the reaction is carried out in 1 to 18
bubble columns.
9. The process of claim 8, wherein the reaction is carried out in 2 to 12
bubble columns.
10. The process of claim 1, wherein the reaction is carried out in at least
two sequentially-connected bubble column reactors in such a way that the
organic carbonate of the formula (II) is metered into the first bubble
column and the aromatic carbonate of the formula (I) is taken off in
liquid form from the last bubble column and the product of the formula
(IV) is taken off at the upper end of the first bubble column.
11. The process of claim 1, wherein a temperature of
100.degree.-250.degree. C. is employed, in the case of a bubble column
cascade, the temperatures in the bubble columns being identical or
different.
12. The process of claim 11, wherein a temperature of 120.degree. to
240.degree. C. is employed.
13. The process of claim 1, wherein a pressure range of 0.05 to 15 bar is
employed, in the case of a bubble column cascade, the pressures in the
individual bubble column being identical or different.
14. The process of claim 13, wherein a pressure range of 0.08 to 13 bar is
employed.
15. The process of claim 1, wherein in the case of a bubble column cascade,
both the pressure and the temperature decrease from the first to the last
bubble column.
16. The process of claim 1, wherein bubble columns having loose packings,
arranged packings or perforated trays are used.
17. The process of claim 1, wherein a bubble column or a bubble column
cascade is combined with one or more downstream residence time vessels.
18. The process of claim 1, wherein the organic carbonate (II) is used in a
mixture with 0-5% by weight based on the weight of (II), of the underlying
alcohol R.sup.3 --OH.
19. The process of claim 18, wherein the amount of the underlying alcohol
in the mixture is 0.1-3% by weight.
20. The process of claim 1, wherein additionally to the starting materials,
an inert solvent evaporating in the reaction mixture or an inert gas is
fed in together with the carbonate of the formula (II) or separately
therefrom at any desired position of the bubble column or bubble column
cascade, which solvent or gas may or may not form an azeotrope with the
product of the formula (IV).
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a continuous process for the preparation of aryl
carbonates from carbonates containing at least one aliphatic ester group
and phenols on the one hand and from alkyl aryl carbonates on the other
hand by catalysed transesterification, the reaction being carried out in
one or more bubble columns.
2. Description of the Related Art
The preparation of aromatic and aliphatic-aromatic carbonic esters
(carbonates) by transesterification, starting from aliphatic carbonic
esters and phenols, is known in principle. This is an equilibrium
reaction, the position of the equilibrium being shifted almost completely
in the direction of the aliphatically substituted carbonates. Therefore,
it is relatively easy to prepare aliphatic carbonates from aromatic
carbonates and alcohols. However, in order to carry out the reaction in
the reverse direction towards aromatic carbonates, it is necessary to
shift effectively the highly unfavourably lying equilibrium, not only
highly active catalysts, but also a favourable procedure having to be
used.
For the transesterification of aliphatic carbonic esters with phenols, a
multiplicity of effective catalysts have been recommended, such as for
example alkali metal hydroxides, Lewis acid catalysts selected from the
group comprising the metal halides (German Offenlegungsschrift 2 528 412
and 2 552 907), organotin compounds (EP 0 000 879, EP 0 000 880, German
Offenlegungsschrift 3 445 552, EP 0 338 760), lead compounds (JP 57/176
932), Lewis acid/proton acid catalysts (German Offenlegungsschrift 3 445
553).
In the known processes, the transesterification is carried out in a
batchwise reactor at atmospheric pressure or under pressure, with or
without an additional separation column. Even with the most highly active
catalysts, reaction times of many hours are required in these cases to
achieve even only average conversion rates of approximately 50% of phenol.
Thus in the batchwise transesterification of phenol with diethyl carbonate
at 180.degree. C. using various organotin compounds, as described in
German Offenlegungsschrift 3 445 552, yields of diphenyl carbonate of an
order of magnitude of more than 20% are only achieved after a reaction
time of approximately 24 hours; in the batchwise transesterification of
phenol and dimethyl carbonate with the aid of organotin catalysts, as
described in EP 0 000 879, the phenol conversion rate after 30 h is 34% of
the theoretical value.
This means that, owing to the unfavourable thermodynamic conditions, the
batchwise transesterification reactions described, even with the use of
highly active catalyst systems, can only be carried out in the sense of an
industrial process highly disadvantageously, since very poor space-time
yields and high residence times with high reaction temperatures are
required.
Such procedures are also particularly disadvantageous since even with
highly selective transesterification catalysts at high temperatures and
with long residence times of many hours, a marked proportion of side
reactions occurs, for example ether formation with elimination of carbon
dioxide.
It was therefore attempted to shift the reaction equilibrium as rapidly as
possible in the direction of the products by adsorption to molecular
sieves of the alcohol resulting in the transesterification (German
Offenlegungsschrift 3 308 921). From the description of this procedure it
appears that, for the adsorption of the reaction alcohol, a large amount
of molecular sieve is required, which exceeds the amount of liberated
alcohol by at least five fold. Furthermore, the molecular sieves used must
be regenerated even after a short time and the conversion rate to the
alkyl aryl carbonate intermediates is relatively low. This process
therefore also appears not to be advantageously industrially and
economically applicable.
A continuous transesterification process for the preparation of aromatic
carbonates in which the reaction is carried out in one or more
multiple-stage sequentially-connected distillation columns is described in
EP-A 0 461 274. In this case, phenols are initially reacted with dialkyl
carbonates to give aryl carbonate mixtures which in the main contain alkyl
aryl carbonates. In a second, preferably downstream, multiple-stage
distillation column, these are then further reacted to give the desired
diaryl carbonate end products. The applicant emphasizes the effectiveness
and the selectivity of its procedure.
Apart from conversion rates and selectivity, the citation of the space-time
yield (STY) serves as a criterion for the evaluation of a process for
those skilled in the art, since it describes the yield of product per unit
of apparatus volume used. By way of the example of the transesterification
of dimethyl carbonate (DMC) with phenol to give methyl phenyl carbonate
(MPC) and diphenyl carbonate (DPC), the applicant of EP 0 461 274 shows a
comparison of the batch mode of operation in an autoclave (Comparative
Example 1) with a mode of operation in a multiple-stage distillation
column (Example 1). In this case, only an increase of the STY from 5 to 8
g of the sum of DPC+MPC/1.h is achieved, as can easily be calculated from
the examples. The STYs are comparatively low in both examples; only the
MPC selectivity increased in the mode of operation in a multiple-stage
distillation column from 94% to 97%. These results are achieved already
under optimal conditions with the best transesterification catalysts at
high temperatures and elevated pressure, so that further improvements do
not appear to be possible.
The further reaction of the alkyl aryl carbonates to give diaryl carbonates
proceeds in the procedure cited, as follows from the examples, in the
sense of a disproportionation reaction. It is thus no wonder that in this
reaction proceeding more readily in comparison to the first
transesterification stage significantly higher STYs are achieved.
For the second transesterification stage, EP 0 461 274 compares the
transesterification of methyl phenyl carbonate (MPC) to give diphenyl
carbonate (DPC) in the batch mode of operation in the autoclave
(Comparative Example 2) with carrying out the transesterification in a
multiple-stage distillation column (Example 11). In this case, the STYs
for DPC calculated from the data given there even show a reduction in the
effectiveness from 144 g of DPC/1.h to 133 g of DPC/1.h. Only the
formation of the by-product anisole occurs to a lesser extent.
Because of these figures and the considerably higher apparatus complexity,
the improvement demonstrated here must be evaluated extremely sceptically.
The aim of an improvement of the transesterification reaction according to
the invention should therefore primarily be an increase of the STYs,
primarily of the transesterification stages with phenol, in which the
selectivity of the overall process should not be reduced.
SUMMARY OF THE INVENTION
Surprisingly, it has now been found that the increase of the STYs succeeds
in a continuously performed transesterification process at very high
selectivity in bubble columns. This was particularly surprising, since
bubble columns are putatively unsuitable reactors for this reaction,
resemble batchwise reactors in their properties and in them, therefore,
longer liquid residence times occur compared with a distillation column,
which increase the risk of formation of by-products. High STYs in the
carbonate transesterification according to the invention are accomplished
in bubble column reactors even at low temperatures and even in operations
in atmospheric pressure. The reactors, which are unusual for this
reaction, are otherwise known, to those skilled in the art, primarily for
absorption processes, for example in exhaust gas purification.
Bubble column reactors are simple apparatuses without stirrers, in which
temperature, pressure and in particular the liquid residence times can be
adjusted in broad ranges, so that a variable procedure is available.
The invention therefore relates to a process for the preparation of an
aromatic carbonate of the formula
R.sup.1 --O--CO--O--R.sup.2 (I)
in which
R.sup.2 denotes phenyl or naphthyl each of which may be monosubstituted to
trisubstituted by straight-chain or branched C.sub.1 -C.sub.4 -alkyl,
straight-chain or branched C.sub.1 -C.sub.4 -alkoxy, cyano and/or halogen,
and
R.sup.1, independently of R.sup.2, assumes the range of meanings of R.sup.2
or denotes straight-chain or branched C.sub.1 -C.sub.6 -alkyl,
by catalysed reaction of 0.1 to 10 mol, preferably 0.2 to 5 mol,
particularly preferably 0.5 to 3 mol, of an organic carbonate having at
least one aliphatic ester group of the formula
R.sup.1 --O--CO--O--R.sup.3 (II)
in which
R.sup.3 denotes straight-chain or branched C.sub.1 -C.sub.6 -alkyl and
R.sup.1 has the above range of meanings,
with 1 mol of a phenolic compound of the formula
R.sup.2 --OX (III)
in which
R.sup.2 has the above range of meanings and
X represents hydrogen or --CO--O--C.sub.1 -C.sub.6 -alkyl having a
straight-chain or branched alkyl group, in the presence of a
transesterification catalyst known per se at 80.degree. to 350.degree. C.,
which is characterized in that the reaction is carried out in a bubble
column reactor or a cascade of at least two bubble columns in such a way
that the phenolic compound of the formula (III) is metered in in liquid
form into the first bubble column and the organic carbonate of the formula
(II) is metered in in the liquid or gaseous state simultaneously into each
individual bubble column, but preferably only into the last bubble column,
in the case of liquid metering, an evaporation of (II) in the bubble
column proceeding, and the reaction products of the formula (I) are taken
off from the last bubble column in liquid form and simultaneously at the
upper end of each individual bubble column, preferably at the upper end of
the first bubble column, the products of the formula
R.sup.3 --OX (IV)
in which R.sup.3 and X have the meaning mentioned, are taken off in gaseous
form.
BRIEF DESCRIPTION OF THE DRAWINGS
Accompanying FIGS. 1 and 2 demonstrate by way of example variants of the
inventive process using several bubble columns. FIG. 3 demonstrates a
variant with only one bubble column which was used for the working
examples.
DETAILED DESCRIPTION OF THE INVENTION
The transesterification by the process according to the invention includes
a plurality of reactions, as the equations below show in generalized form
(Alk=alkyl; Ar=aryl ):
Alk--O--CO--O--Alk+Ar--OH.fwdarw.Alk--O--CO--O--Ar+Alk--OH (Equation 1)
Alk--O--CO--O--Ar+Ar--OH.fwdarw.Ar--O--CO--O--Ar+Alk--OH (Equation 2)
2Ar--OCO--O--Alk.fwdarw.Ar--OCO--O--Ar+Alk--OCO--O--Alk (Equation 3)
In the formation of a diaryl carbonate, the transesterification of the
aliphatic ester groups to the aromatic ester groups proceeds in two
stages, an alkyl aryl carbonate being proceeded through according to
equation 1 as a product of the first transesterification stage.
Equation 3 further shows a disproportionation reaction in which both the
symmetrical dialkyl carbonate and the desired symmetrical diaryl carbonate
result from a mixed alkyl aryl carbonate. It is further possible to obtain
the alkyl aryl carbonate as the desired reaction product, that is
essentially only to operate the first transesterification stage. It is yet
further possible to also obtain asymmetrical diaryl carbonates by use of
mixtures of different phenols.
Dialkyl carbonates having identical or different aliphatic ester groups
having straight-chain or branched C.sub.1 -C.sub.6 -alkyl are used. Such
dialkyl carbonates are known to those skilled ill the art and can be
prepared by known methods. For economic reasons, symmetrical dialkyl
carbonates are generally used as starting material.
Straight-chain or branched C.sub.1 -C.sub.6 -alkyl is, for example, methyl,
ethyl, propyl, isopropyl, butyl, isobutyl, pentyl or hexyl, preferably
methyl or ethyl, particularly preferably methyl.
Straight-chain or branched C.sub.1 -C.sub.4 -alkoxy is, for example,
methoxy, ethoxy, propoxy, isopropoxy, butoxy or isobutoxy, preferably
methoxy.
Halogen is, for example, fluorine, chlorine or bromine, preferably fluorine
or chlorine, particularly preferably chlorine.
The aromatic ester group can be derived from a phenol or a naphthol,
preferably from a phenol and can be monosubstituted to trisubstituted in
the manner stated, preferably monosubstituted or disubstituted,
particularly preferably monosubstituted. The cyano substituent generally
occurs only singly as a substituent. The process according to the
invention has high particular importance for the transesterification of
unsubstituted phenol.
Phenols which can be used according to the invention and which are included
under the formula (III) when X represents hydrogen are, for example,
unsubstituted phenol, o-, m- or p-cresol, o-, m- or p-chlorophenol, o-, m-
or p-ethylphenol, o-, m- or p-propylphenol, o-, m- or p-methoxyphenol,
2,6-dimethylphenol, 2,4-dimethylphenol, 3,4-dimethylphenol, 1-naphthol and
2-naphthol.
Phenolic compounds which can preferably be used are therefore generally
those of the formula
R.sup.12 --OH (V)
in which
R.sup.12 denotes phenyl or phenyl monosubstituted by C.sub.1 -C.sub.4
-alkyl, C.sub.1 -C.sub.4 -alkoxy or chlorine.
Among these, unsubstituted phenol is particularly preferred.
The organic carbonates having at least one aliphatic ester group preferably
used are symmetrical dialkyl carbonates of the formula
R.sup.3 --O--CO--O--R.sup.3 (VI)
in which
R.sup.3 has the meaning given.
Dialkyl carbonates which can be used according to the invention are, for
example, dimethyl carbonate, diethyl carbonate, dipropyl carbonate,
dibutyl carbonate and dihexyl carbonate. Dialkyl carbonates which can
preferably be used are dimethyl and diethyl carbonate, particularly
preferably dimethyl carbonate (DMC).
The organic carbonate (II) having at least one aliphatic ester group can be
used as such in the process according to the invention. However, it is
possible, and represents a preferred variant, to use this organic
carbonate in a mixture with small amounts of the underlying alcohol
R.sup.3 --OH. The alcohol R.sup.3 --OH occurs as an elimination product in
the process according to the invention and signifies the special case of
the formula (IV) with X=H. The elimination products carbonate
(X=--CO--O--C.sub.2 -C.sub.6 -alkyl) and alcohol (X=H) therefore do not
need to be completely separated for return of the carbonate to the process
according to the invention; this signifies an energetic advantage. The
amount of the alcohol permissible in the mixture with the carbonate is
0-5% by weight, preferably 0.1-3% by weight, particularly preferably
0.15-2% by weight, based on the amount of carbonate used. The lower limit
zero indicates the operation with pure carbonate.
Diaryl carbonates which can be prepared according to the invention are, for
example, diphenyl carbonate, the symmetrically and asymmetrically
substituted isomeric biscresyl carbonates, the symmetrically and
asymmetrically substituted isomeric bis(chlorophenyl) carbonates, the
symmetrically and asymmetrically substituted isomeric bis(methoxyphenyl)
carbonates, the symmetrically and asymmetrically substituted isomeric
bis(ethoxyphenyl) carbonates, bis(2,6-dimethylphenyl) carbonate,
bis(2,4-dimethylphenyl) carbonate, di-1-naphthyl carbonate and
di-2-naphthyl carbonate, furthermore other asymmetrically substituted
diaryl carbonates, for example the isomeric cresyl phenyl carbonates, the
isomeric chlorophenyl phenyl carbonates, the isomeric methoxyphenyl phenyl
carbonates, the isomeric naphthyl phenyl carbonates and 1-naphthyl
2-naphthyl carbonate.
Diaryl carbonates which can preferably be prepared according to the
invention are those of the formulae
R.sup.15 --OCOO--R.sup.12 (VII)
and
R.sup.12 --OCOO--R.sup.12 (VIII)
in which
R.sup.12 and R.sup.15, independently of each other, have the range of
meanings given above for R.sup.12.
A diaryl carbonate which can be particularly preferably prepared is
diphenyl carbonate.
Alkyl aryl carbonates which can be prepared according to the invention are,
for example, C.sub.1 -C.sub.6 -alkyl phenyl carbonates, such as methyl
phenyl carbonate, ethyl phenyl carbonate, propyl phenyl carbonate, butyl
phenyl carbonate and hexyl phenyl carbonate, C.sub.1 -C.sub.6 -alkyl (o-,
m-, p-cresyl) carbonates, such as methyl o-cresyl carbonate, methyl
p-cresyl carbonate, ethyl o-cresyl carbonate, ethyl p-cresyl carbonate,
C.sub.1 -C.sub.6 -alkyl (o-, m-, p-chlorophenyl) carbonates, such as
methyl p-chlorophenyl carbonate or ethyl p-chlorophenyl carbonate and
analogous compounds. Alkyl aryl carbonates which can be particularly
preferably prepared are methyl phenyl carbonate and ethyl phenyl
carbonate, very particularly preferably methyl phenyl carbonate.
The bubble column reactors which can be used in the process according to
the invention are the following types: simple bubble columns, cascades of
simple bubble columns, bubble columns having internals and cascades of
these bubble columns, such as: bubble columns having parallel chambers,
cascade bubble columns, bubble columns having packings, bubble columns
having static mixers, pulsed sieve-tray bubble columns, and other bubble
column reactors known to those skilled in the art (H. Gerstenberg, Chem.
Ing. Tech. 61 (1979) No. 3, p. 208-216; W. D. Deckwer, Reaktionstechnik in
Blasensaulen [Reaction Technique in Bubble Columns], Otto Salle Verlag
(1985)).
In the preferred embodiment, the bubble column reactors or cascades of
bubble column reactors below are used: simple bubble columns, cascade
bubble columns, bubble columns having parallel chambers and bubble columns
having static mixers or packings.
In a further preferred embodiment, combinations both of the individual
bubble column reactors in a cascade of bubble columns and in a cascade
bubble column can also be used.
To maintain as homogeneous as possible a bubble flow through the liquid,
distribution and redispersion elements can be mounted in the bubble column
reactor along the longitudinal axis.
The fixed redispersion elements which are used are single-hole trays,
perforated plates, sieve trays and other internals known to those skilled
in the art which, when backmixing is effectively avoided, enable the
counter-flow of gas phase and liquid phase.
In the individual cascade bubble column reactors, after the first
dispersion of the gas phase, a further 0 to 20, preferably 1 to 15,
redispersion elements can be present. In this case, a bubble column having
0 redispersion elements signifies the special case of a simple bubble
column. The total number of the redispersion elements in a cascade of
bubble columns can thus be 100, preferably 75, particularly preferably up
to 60.
In the counter-current flow of the liquid phase and gas phase in cascade
bubble columns, the liquid can either flow through the dispersion elements
or flow through internal and/or external overflow pipes to the bubble
column sections situated beneath.
For the initial dispersion of the gaseous carbonate of the formula (II) in
the liquid phase at metering, conventional apparatuses can be used, such
as porous sinter plates, perforated plates, sieve trays, push-in pipes,
nozzles, ring spargers and other dispersion apparatuses known to those
skilled in the art.
Within a bubble column, or, in the case of the use of a cascade of bubble
columns, also within an individual bubble column, various types of the
abovementioned dispersion elements can be present simultaneously, that is,
for example, fixed internals as well as packings.
The liquid holdup in the bubble column reactors is more than 40%,
preferably more than 50%, and particularly preferably more than 75%, of
the available volume.
The gas velocity, based on the empty reactor cross-section, is 0.1 to 100
cm/s, preferably 1 to 50 cm/s and particularly preferably 2 to 30 cm/s.
The slenderness ratio of the bubble column reactors (ratio of length to
diameter) is 1 to 30, preferably 1-20.
For the case that bubble column reactors having parallel chambers are used,
the ratio of length to overall diameter of the bubble column can deviate
from these figures, since here the individual chambers are to be taken
into account.
For the supply of heat to the bubble columns, external heaters are
suitable, such as jacket heaters, heat exchangers for liquids taken off
intermediately or internal heat exchangers, such as parallel single tubes,
transverse tube bundles, longitudinal tube bundles, spiral pipe coils,
helical pipe coils, jacketed draught tubes and other heat exchange
apparatuses known to those skilled in the art as prior art. In a preferred
embodiment, the internal heat exchangers can additionally assume
directional functions for the liquid flow and the gas dispersion.
To separate off the more readily volatile components from the liquid phase
produced at the lower end, a stripping column can be installed according
to the prior art. In the same way, to purify the gas phase, produced from
dialkyl carbonate and the relevant alcohol, from the aromatic hydroxyl
compound and the transesterification products alkyl aryl carbonate and
diaryl carbonate, the upper end of the bubble column can be equipped with
an enrichment column.
In a further procedure, additionally to the starting materials, a solvent
inert under the reaction conditions which evaporates in the bubble column
or gas can be fed into the apparatus at any desired position. Such inert
solvents are, for example, hydrocarbons, such as hexane, heptane,
i-octane, methyl-cyclopentane, cyclohexane, methylcyclohexane, toluene,
xylenes, chlorobenzenes, Tetralin, Dekalin etc. Inert gases which are
useful are, for example, carbon dioxide, nitrogen, noble gases etc. These
inert solvents and gases can also be metered in together with the gaseous
carbonate or the carbonate to evaporate in the bubble column and can be
varied in a broad concentration range.
In some embodiments it can be expedient also to meter the pure inert gas or
solvent into one or more bubble columns.
For the case when DMC is used as aliphatic carbonate, it can be
advantageous to use an inert solvent which forms an azeotrope with
methanol and preferentially removes this from the bubble column. The
removal of methanol from the equilibrium promotes the continuation of the
process according to the invention.
In FIGS. 1 and 2, different exemplary embodiments of the invention are
shown. Numbers and letters quoted in the text refer to these figures.
Therein, the process according to the invention is preferably carried out
using 1 to 18, particularly preferably 2 to 12 bubble column reactors, the
lower limit 1 signifying carrying out the process in a single bubble
column.
In the preferred embodiment, a cascade of cascade bubble column reactors is
used (cascade bubble columns). In FIGS. 1 and 2, exemplary operations with
3 bubble column reactors (A, B and C) are depicted, in which the operation
according to the invention is not intended to be restricted to these
examples. D and E signify residence time vessels described later for the
completion of the reaction and stripping sections of columns for mass
separations, respectively.
The reaction component of the formula (III) metered into the first bubble
column (A) can optionally be preheated in an upstream heater element to
the intended reaction temperature. It is preferably introduced into the
bubble column at the upper end in liquid form via line (1).
The liquid phase to be taken off from the particular bubble column is taken
off at the lower end and metered in again at the upper end to the
respective following bubble column B or C via the lines (2), (3) or (4).
The regulation of the desired filling level in the continuously operated
bubble column reactors is carried out according to the prior art.
When a bubble column cascade is used, the gas phase (II) can be fed through
the continuously running liquid stream (III)+(I) either in cross-flow
(FIG. 1) or preferably in counter-current (FIG. 2).
Cross-flow denotes in this case that the starting materials of the formula
(II) are each metered into every bubble column reactor via the lines (12),
(13), (5) (FIG. 1) and are each taken off again at the upper end of each
bubble column via the lines (8), (7) and (6) (FIG. 1), that is the
starting materials of the formula (II) flow through the bubble column
reactors transversely to the direction of flow of the liquid phase
(III)+(I). The total amount of the starting materials of the formula (II)
metered in can in this case be apportioned as desired to the individual
bubble column reactors. In the particular bubble column reactor, in this
case, the counter-current mode of operation of liquid phase and gas phase
is preferably realized.
The counter-current mode of operation preferably to be used (FIG. 2)
denotes that the starting materials of the formula (II) are metered into
the last bubble column reactor (in FIG. 2, reactor C), continuously
conducted in the opposite direction to the liquid phase running from the
first bubble column reactor to the last reactor (C in FIG. 2) and excess
starting material (II) and product formed (IV) are taken off at the upper
end of the first bubble column reactor (A in FIG. 2). If (II) and (IV)
form an azeotrope, as in the case DMC/methanol, it can be expedient to
take off some of such an azeotrope at the upper end of intermediate
reactors as well.
The starting materials of the formula (II) and the inert compound
optionally added can in both cases be either metered in in the liquid
state and evaporated by the liquid phase present or, preferably,
evaporated in an upstream apparatus and introduced in the gaseous state
into the respective bubble column.
It is furthermore also possible to have the starting materials of the
formula (II) flow partly in cross-flow and partly in counter-current to
the liquid phase (III)+(I).
The reaction products of the formula (IV) to be taken off at the upper end
of the respective bubble column can be taken off, for example, directly in
the gaseous state via (6'), (7') and (8').
It is in this case possibly advantageous, by suitable dephlegmation or/and
by an attached column to separate off previously higher-boiling reaction
constituents, for example products of the formula (I) or starting
materials of the formula (III), and to return them to the respective
bubble column. The products of the formula (IV) can, for example, for this
purpose be introduced without condensation to a suitable separation
apparatus. In the case of the reaction of dimethyl carbonate with phenol,
this could be a pressure distillation column for separating the dimethyl
carbonate/methanol mixture produced, in order to keep as little as
possible DMC in the top product of the separation column. The dimethyl
carbonate produced in this case, which possibly still contains small
amounts of methanol, can be returned as starting material of the formula
(II) to the transesterification process.
In the same way, it is possible to take off the products of the formula
(IV), if required after separating off higher-boiling reaction
constituents, as described above, and to condense them. A purification and
fractionation of the product stream can then be carried out in a suitable
manner known to those skilled in the art.
The product stream to be taken off in the liquid state at the last reactor,
for example C in FIGS. 1 and 2, can be separated off if required in a
downstream stripping section (E in FIGS. 1 and 2) from low-boiling
constituents, for example starting materials of the formula (II) or the
products of the formula (IV), which are then returned to the reactors, for
example the last bubble column of the cascade (C). The product stream
taken off in the liquid state can be worked up and purified by
conventional methods, for example by distillation.
In a particularly preferred embodiment, the product stream to be taken off
in the liquid state is passed into 1 to 5, preferably 1 to 3 downstream
reactors, a further reaction in the sense of equation 2 and/or 3 being
able to proceed there. These reactors are, for example, additional bubble
columns, stirred tanks or a reaction distillation which are treated with
one or more inert compounds, gaseous under the reaction conditions (line
(9), possibly via a preheater/evaporator). In FIGS. 1 and 2, this mode of
operation is illustrated, simplified for clarity by a single bubble column
reactor (D), in which the mode of operation according to the invention is
not intended to be restricted hereby.
In this case, the aromatic carbonate of the formula (I) is taken off at
(11) and the volatile reaction product produced in reactor D is taken off
together with the gaseous compounds at (10').
The respective last residence time vessel D can optionally have a
downstream stripping section by means of which low-boiling products of the
formulae (IV)+(II) and/or unreacted starting materials of the formula
(III) are completely or partly returned to this residence time vessel D.
In the same way, it is possibly advantageous to separate off the volatile
reaction products of the formula (IV), to be taken off at the upper end of
the first residence time vessel D for example via (10'), from
higher-boiling products of the formula (I) or starting materials of the
formula (III) via an enrichment and/or dephlegmator section attached there
via the line (10) and to return these to D.
The gaseous compounds in the meaning just mentioned of the invention which
are used are for example superheated phenol, inert gases alone, such as
nitrogen, noble gases, carbon dioxide, C.sub.1 -C.sub.12 -alkanes, cyclic
alkanes, such as cyclohexane, Dekalin, aromatic hydrocarbons, such as
benzene, toluene, xylenes, cumene, mesitylene, and mixtures of inert gases
or mixtures of phenol with inert gas. In the preferred embodiment, easily
condensable compounds, such as phenol, toluene, mesitylene, Dekalin, alone
or as mixtures, are used. However, for the case that only the first
transesterification stage according to equation (1) is desired, it is
entirely possible to introduce dialkyl carbonate, optionally in a mixture
with inert gas, into all or individual bubble columns and into the
residence time vessels. Such an inert gas can advantageously in turn be an
azeotrope-former for alkanol to be discharged.
The product stream taken off in the liquid state at the bubble column
reactor or, possibly, at the last reactor of a bubble column cascade after
the 1st transesterification stage, which contains the products of the
formula (I) particularly according to equation (1), to a lesser extent
also according to equations (2) and (3) can, in a further particular
embodiment of the invention, with or without intermediate storage in
suitable vessels, be metered in place of the starting material of the
formula (III) back into the bubble column reactor or, possibly, into the
1st bubble column of a bubble column cascade, in order to carry out or
complete the 2nd transesterification stage according to equation (2) or a
disproportionation according to equation (3). This is also optionally
possible repeatedly, the feed of the second starting material of the
formula (II) also, optionally, being able to be omitted and replaced by
inert compounds gaseous under reaction conditions. To continuously carry
out such a mode of operation, for example, either at least two storage
vessels or one storage vessel having at least two chambers are necessary,
the product from the running reaction being fed into the 1st chamber and
the starting material for the running reaction being taken off from the
2nd chamber. When one chamber is emptied or one chamber is filled, the 2nd
chamber is used for receiving the product from the bubble column reactor
or from the last reactor of a bubble column cascade and the 1st chamber is
used for feeding the starting material into the bubble column reactor or
into the bubble column cascade.
Alternatively, in a further embodiment, a further treatment of the liquid
reaction product from the 1st transesterification stage can be carried
out, as for example in FIGS. 1 and 2 the outflow of line (4) to reactor
(C), in a multiple-stage distillation apparatus in the meaning of EP 0 461
274, a further reaction being able to proceed there according to equation
(2) and/or (3).
In a further variant, the residence time vessel D is designed in the form
of a distillation apparatus which is operated in the meaning of a
"reaction distillation", that is, simultaneously to the proceeding
reaction, a distillation of the participating substances is carried out.
The essential characteristics of a "reaction distillation" in the meaning
of the invention are the following: the as yet unreacted alkyl aryl
carbonate intermediate from the 1st transesterification stage is
substantially prevented, by a specially selected temperature gradient in
the distillation apparatus, from leaving the reaction section of the
reactor at the top or at the bottom. The readily volatile reaction
products of the formula (IV) are taken off at the head of the column, the
poorly volatile reaction product, here the diaryl carbonate (2nd
transesterification stage), is taken off at the foot of the column. Any
excess phenol possibly present can be taken off together with the diaryl
carbonate end products at the foot of the distillation apparatus or
together with the low-boiling products at the head of the apparatus.
The reactor designated as a "reaction column" is composed of a column-like
tube to which is applied a temperature profile which includes a
temperature range increasing from top to bottom of 60.degree. to
320.degree. C., preferably 65.degree. to 305.degree. C. and particularly
preferably 65.degree. to 250.degree. C. To establish the temperature
gradients in the individual sections of the column-like reactor, these
sections can be provided with insulation or thermostatting. The
thermostatting in this case can signify heating or cooling as required.
The reaction column can be expanded or contracted in various sections of
its overall length, in correspondence with the gas and liquid loadings and
the required residence times.
Fixed internals are preferred for the central part of the reaction column,
the reaction region, and in contrast, loose packings and fixed packings
are preferred for the parts in which separations take place.
At the lower end of the reaction column are arranged one or more
evaporators, optionally separated by adiabatically insulated column parts.
These evaporators can be arranged inside or outside the column. In an
industrial embodiment, equipment conventional in the technology, such as
circulation evaporators, falling film evaporators and spiral tube
evaporators is used.
Above the evaporator zone, in the central region designated as "reaction
zone", fixed internals or, for example, bubble-cap trays are preferably
used. The theoretical number of plates in this region is 1 to 50,
preferably 1 to 25 and particularly 1 to 15.
Above this region in turn, the column is equipped with further loose
packings, packings or internals particularly suitable for mass separations
by distillation. At the upper end of the column an enrichment section is
preferably arranged, by means of which a specific reflux to the column can
be established.
The reaction column is operated in such a way that the product stream from
the 1st transesterification stage, taken off in the liquid state from the
bubble column reactor or the bubble column cascade, is metered in in the
liquid state above the "reaction zone". This stream passes through the
"reaction zone" and is there partly converted into diaryl carbonate
according to equations (2) and (3) and the as yet unreacted reactants are
transported in the gaseous state with the aid of the described evaporators
back to the reaction zone and the upper parts of the column. These
condense there and react afresh to give the diaryl carbonate end product.
The diaryl carbonate end product is enriched in the bottom region of the
column as the highest boiling reaction component and is there fed out
together with any homogeneously dissolved catalyst and small amounts of
alkyl aryl carbonate and aromatic hydroxyl compound.
The readily volatile reaction products of the formula (IV) are taken off at
the head of the column. The phenols of the formula (III), present in
excess or unreacted, can be fed out at the foot of the column with the
diaryl carbonate end product of the formula (I) or, in a preferred mode of
operation, with the low-boiling products at the head of the column.
In a further procedure, the product stream to be taken off in the liquid
state can be passed into 1 to 5, preferably 1 to 3, downstream residence
time vessels D, optionally stirred or treated with inert gas, further
reactions according to equation 2 and/or equation 3 being able to proceed
there. In this case, the aromatic carbonate of the formula (I) is taken
off at (11) and volatile reaction products produced in D are taken off at
(10) or (10').
To mix the reaction components, the stirred vessels to be used according to
the invention are equipped with agitators usable therefor. Such stirrers
are known to those skilled in the art. The following can be mentioned by
way of example: disc stirrers, impeller stirrers, propeller stirrers,
paddle stirrers, MIG stirrers and Intermig stirrers, tubular stirrers and
other hollow stirrer types. Preferred stirrers are those which permit an
effective mixing of gases and liquids, for example hollow stirrers, such
as tubular stirrers and triangular stirrers, propeller stirrers, turbine
stirrers etc.
For improved mixing, the stirred vessels can preferably be provided with
flow-breaker internals. These flow breakers can simultaneously be designed
to be thermostattable for introducing heat into the reactor or conducting
heat away from the reactor.
Those modes of operation and embodiments of the invention are preferably
used in which additional residence time vessels are used in the form of
columns or stirred tanks.
Possible embodiments in terms of apparatus for carrying out the process
according to the invention are the following, the listing being in no way
exhaustive:
a bubble column,
a bubble column having a residence time vessel in the form of a stirred
tank and/or a distillation column,
a bubble column having a plurality of residence time vessels in the form of
stirred tanks and/or distillation columns,
a cascade of two or more bubble columns,
a bubble column cascade of two or more bubble columns having a residence
time vessel in the form of a stirred tank or a distillation column,
a cascade of two or more bubble columns having a plurality of residence
time vessels in the form of stirred tanks and/or distillation columns,
in all cases bubble columns being able to be used without or with internals
of the type mentioned.
The heat of reaction necessary for the reaction can be introduced with the
starting materials. However, it is preferred to introduce additional
energy into the reactor for example via a jacket heating and/or by
internal heating elements.
The further work-up of the reaction products of the formula (I), taken off
in the liquid state via line (11), which can contain excess phenolic
compound (III) and, possibly, further, a homogeneous dissolved catalyst,
can be carried out by conventional methods, for example by distillation.
In a preferred embodiment, if a titanium compound, for example titanium
tetraphenolate, is used as catalyst, this can be separated off from the
reaction product of the 2nd transesterification stage before the work-up
by distillation of the liquid reaction product by crystallization and
subsequent filtration or sedimentation.
For the separation, the liquid reaction mixture is cooled for this purpose
to a temperature of 40.degree. to 120.degree. C., preferably 50.degree. to
100.degree. C., particularly preferably 60.degree. to 90.degree. C., this
mixture having to remain liquid. The sedimented titanium-containing
precipitate can then be separated off. The remaining reaction mixture
contains residual titanium amounts of less than 100 ppm. The catalyst thus
separated off can be returned, if required without further purification,
to the process.
By the cooling according to the invention of the reaction mixture and
separating off of the sedimented, titanium-containing precipitate, in a
surprisingly simple operation, a reaction mixture is obtained which can be
worked up both by crystallization and by distillation under conditions
conventional per se for isolating the aromatic carbonate, without the fear
of loss of yields. Special reaction conditions and special precautionary
measures which would be required by the presence of the catalyst are
therefore no longer required.
The separation of the titanium catalyst can optionally also be carried out
even after the first transesterification stage (after reactor (C) in FIGS.
1 and 2), if, for example, an alkyl aryl carbonate is desired or another
catalyst is intended for the 2nd transesterification stage.
The transesterification catalysts to be used and known as such are
preferably introduced in dissolved or suspended form into the bubble
column reactor or the bubble column cascade together with the starting
materials of the formula (III) to be metered in in the liquid state.
Alternatively, the catalyst can also be metered in separately or dissolved
or suspended in a small amount of the starting material of the formula
(III) or in a suitable inert solvent, see above, external to the system.
In the case of the use of heterogeneous catalysts, these can also be used
directly in an immobile state in the bubble column reactor or in the
bubble column cascade.
A suitable filter apparatus must prevent the discharge of the catalysts in
this case.
It is important that a catalyst is present on at least 2 distribution
elements in a cascade bubble column or in at least 2 bubble columns in a
bubble column cascade.
In the case of the use of non-immobile catalysts, it is possible to return,
as described above, the catalyst back to the reaction process, after
partial or complete separation from the products or starting materials, if
required a portion of the catalyst corresponding to the amount of catalyst
deactivated being separated off and replaced by fresh catalyst.
The process according to the invention is carried out at temperatures in
the liquid phase from 80.degree. to 350.degree. C., preferably at
100.degree. to 250.degree. C. and particularly preferably at temperatures
from 120.degree. to 240.degree. C. The liquid phase temperature in the
bubble column reactors should not exceed the evaporation temperature of
the phenolic compound of the formula (III) used or of the phenolic
solution used. It can therefore be advantageous to carry out the
transesterification according to the invention in the region of the bubble
column reactors not only at atmospheric pressure but also at elevated or
reduced pressure in the range from 10 mbar to 20 bar. A preferred pressure
range is between 0.05 and 15 bar, and a particularly preferred pressure
range is between 0.08 and 13 bar. In this case it can be expedient to
operate the individual reactors of a cascade each at individual pressures.
With the pressures the temperature can be varied if required in the
individual bubble column reactors of a cascade. In a preferred embodiment,
for example, both pressure and temperature can decrease from the 1st to
the last bubble column reactor.
Catalysts which are useful for the process according to the invention and
which can be identical for all phases of the process according to the
invention are known in the literature. Such catalysts are, for example,
hydrides, oxides, hydroxides, alcoholates, amides or salts of
alkali(alkaline earth) metals, such as lithium, sodium, potassium,
rubidium, caesium, magnesium and calcium, preferably of lithium, sodium,
potassium, magnesium and calcium, particularly preferably of lithium,
sodium and potassium (U.S. Pat. No. 3,642,858, U.S. Pat. No. 3,803,201, EP
1082). For the case of the use of the alcoholates, these can also be
formed according to the invention in situ by use of the elemental alkali
metals and the alcohol to be reacted according to the invention. Salts of
the alkali(alkaline earth) metals can be those of organic or inorganic
acids, such as of acetic acid, propionic acid, butyric acid, benzoic acid,
stearic acid, carbonic acid (carbonates or hydrogen carbonates), of
hydrochloric acid, hydrobromic or hydriodic acid, nitric acid, sulphuric
acid, hydrofluoric acid, phosphoric acid, hydrocyanic acid, thiocyanic
acid, boric acid, stannic acid, C.sub.1 -C.sub.4 -stannonic acids or
antimonic acids. Preferably, compounds of the alkali(alkaline earth)
metals which are useful are the oxides, hydroxides, alcoholates, acetates,
propionates, benzoates, carbonates and hydrogen carbonates, particularly
preferably used being hydroxides, alcoholates, acetates, benzoates or
carbonates.
Such alkali(alkaline earth) metal compounds (optionally formed in situ from
the free alkali metals) are used in amounts of 0.001 to 2% by weight,
preferably 0.005 to 0.9% by weight, particularly preferably 0.01 to 0.5%
by weight, based on the reaction mixture to be reacted.
Further catalysts which can be used according to the invention are Lewis
acid metal compounds such as AlX.sub.3, TiX.sub.3, UX.sub.4, TiX.sub.4,
VOX.sub.3, VX.sub.5, ZnX.sub.2, FeX.sub.3 and SnX.sub.4, in which X
represents halogen, acetoxy or aryloxy (German Offenlegungsschrift 2 528
412, 2 552 907), for example titanium tetrachloride, titanium
tetraphenoxide, titanium tetraethoxide, titanium tetraisopropylate,
titanium tetradodecylate, tin tetraisooctylate and aluminium
triisopropylate, furthermore organotin compounds of the general formula
(R.sup.4).sub.4-x --Sn(Y).sub.x, in which Y represents a radical
OCOR.sup.5, OH or OR, where R.sup.5 denotes C.sub.1 -C.sub.12 -alkyl,
C.sub.6 -C.sub.12 -aryl or C.sub.7 -C.sub.13 -alkylaryl and R.sup.4,
independently of R.sup.5, can assume the range of meanings of R.sup.5 and
x denotes an integer from 1 to 3, dialkyltin compounds having 1 to 12 C
atoms in the alkyl radical or bis(trialkyltin) compounds, for example
trimethyltin acetate, triethyltin benzoate, tributyltin acetate,
triphenyltin acetate, dibutyltin diacetate, dibutyltin dilaurate,
dioctyltin dilaurate, dibutyltin adipate, dibutyl dimethoxytin,
dimethyltin glycolate, dibutyl diethoxytin, triethyltin hydroxide,
hexaethylstannoxane, hexabutylstannoxane, dibutyltin oxide, dioctyltin
oxide, butyltin triisooctylate, octyltin triisooctylate, butylstannonic
acid and octylstannonic acid in amounts of 0.001 to 20% by weight (EP 879,
EP 880, EP 39 452, German Offenlegungsschrift 3 445 555, JP 79/62 023),
polymeric tin compounds of the formula --[--R.sup.4,R.sup.5 Sn--O--]--,
for example poly[oxy (dibutyl stannylene)], poly[oxy (dioctylstannylene)],
poly[oxy(butylphenylstannylene)] and poly[oxy (diphenyl-stannylene)]
(German Offenlegungsschrift 3 445 552), polymeric hydroxystannoxanes of
the formula --[R.sup.4 Sn(OH)--O--]--, for example
poly(ethylhydroxystannoxane), poly(butyl-hydroxystannoxane),
poly(octylhydroxystannoxane), poly(undecylhydroxystannoxane) and
poly(dodecylhydroxystannoxane) in amounts of 0.001 to 20% by weight,
preferably from 0.005 to 5% by weight, based on dicarbonate (DE 4 006
520). Other tin compounds which can be used according to the invention are
Sn(II) oxide or have the formula
X.sup.1 --Sn(R.sup.4).sub.2 --O--Sn(R.sup.4).sub.2 --X.sup.2(IX)
in which
X.sup.1 and X.sup.2, independently of each other, denote OH, SCN, OR.sup.4,
OCOR.sup.4 or halogen and
R.sup.4 denotes alkyl, aryl (EP 338 760).
Other catalysts which can be used according to the invention are lead
compounds, optionally together with triorganophosphanes, with a chelate
compound or with an alkali metal halide, for example Pb(OH).sub.2 --2PbCO,
Pb(OCO--CH.sub.3).sub.2, Pb(OCO--CH.sub.3).sub.2 --2LiCl,
Pb(OCO--CH.sub.3).sub.2 2PPh.sub.3 in amounts of 0.001 to 1, preferably
from 0.005 to 0.25 mol per mol of carbonate (JP 57/176 932, JP 01/093
580), other lead (II) and lead (IV) compounds, such as PbO, PbO.sub.2, red
lead oxide plumbites (PbO.sub.2.sup.2-) and plumbates (PbO.sub.3.sup.2-)
(JP 01/093 560), iron(III) acetate (JP 61/172 852), furthermore copper
salts and/or metal complexes, for example of alkali metal, zinc, titanium
and iron (JP 89/005 588), combinations of Lewis acids and proton acids
(German Offenlegungsschrift 3 445 553) or element compounds of Sc, Cr, Mo,
W, Mn, Au, Ga, In, Bi, Te and lanthanides (EP 338 760).
Furthermore, heterogeneous catalyst systems are usable in the process
according to the invention. These are for example mixed oxides of silicon
and titanium which can be prepared by collective hydrolysis of silicon
halides and titanium halides (JP 54/125 617) and titanium dioxides with a
high BET surface area >208 m.sup.2 /g (German Offenlegungsschrift 4 036
594).
Catalysts which can preferably be used in the process according to the
invention are tin compounds, titanium compounds and zirconium compounds
and the abovementioned alkali metal compounds and alkaline earth metal
compounds, catalysts which are particularly preferably usable are
organotin compounds and titanium tetra-alcoholates and tetraphenolates.
The amounts of catalyst to be used are 0.01 to 10 mol %, preferably 0.05 to
5 mol % and particularly preferably 0.01 to 2 mol %, based on the phenol
component or alkyl aryl carbonate component used and can sometimes differ
from the amounts mentioned in the literature.
The following examples are intended to describe the present invention
concretely, it not being intended to be restricted to these examples.
EXAMPLES
Example 1
(For equipment see FIG. 3; it depicts an embodiment having only one bubble
column. The reference numbers have the meaning given above, in which
metering is performed via line (2) not as in FIGS. 1 and 2 into the next
bubble column, but as the reaction mixture is taken off).
For this example, a bubble column was used (1=60 cm, d=4.5 cm having 10
perforated plates for dispersing the gas phase) having an internal volume
of 950 ml, provided with a heating jacket and heatable by an oil
thermostat. The metering of the liquid phase was performed at the upper
end of the bubble column via a heated line and the takeoff was performed
at the bottom end via a heated height-adjustable siphon. The gas phase was
fed in at the lower end of the bubble column via a glass sinter plate and
taken off at the head via a column 30 cm long filled with Raschig rings
and having an attached column head which permitted the establishment of a
reflux to the bubble column reactor.
The bubble column was filled with 850 ml of phenol and the reactor jacket
was thermostatted with oil to 180.degree. C. Via a heated pump, 500 g/h of
a mixture of 97.8% by weight of phenol and 2.2% by weight of titanium
tetraphenolate (liquid phase) were metered in continuously at the upper
end of the bubble column reactor and, at the same time, 500 g/h of
dimethyl carbonate (DMC), which was continuously evaporated in an
electrically heated tube, was metered in at the lower end. After 4 h, the
reaction was in equilibrium, that is the composition of the gas phase and
the liquid phase no longer changed. At the reactor outlet, 557 g/h of
product mixture containing 65.7 g/h of methyl phenyl carbonate (MPC) and
13.5 g/h of diphenyl carbonate (DPC) were taken off via the siphon. The
rest making up 100% was phenol, little dimethyl carbonate and catalyst. At
the upper end of the bubble column, a product mixture of methanol and DMC
was taken off via the attached column. From this there results a
space-time yield for the formation of MPC and DPC of 83.0 g/1 h. The
selectivity with respect to the formation of aromatic carbonates was
>99.9%.
Example 2
In the equipment described in Example 1 and under the reaction conditions
specified there, 750 g/h of a mixture of 98.6% by weight of phenol and
1.4% by weight of octylstannonic acid were fed in continuously at the
upper end of the bubble column and 750 g/h of DMC at the lower end of the
bubble column. After approximately 3 h, the reaction was in equilibrium.
793 g/h of liquid product mixture containing 105.6 g of MPC and 23 g/h of
DPC were continuously taken off and at the upper end of the bubble column
a mixture of methanol and DMC was taken off. This corresponds to a
space-time yield for MPC and DPC of 135 g/1 h. The selectivity was >99.9%.
Example 3
For this example, a bubble column of 150 cm in length and 2.8 cm in
diameter (923 ml internal volume) and having a packing of 3.times.3 mm V4A
stainless steel wire mesh spirals was used. The reactor jacket was heated
to 180.degree. C. and the bubble column was filled with 600 ml of phenol.
Analogously to Examples 1 and 2, 250 g/h of phenol were metered in
together with 1.4% by weight of octylstannonic acid and 250 g/h of DMC.
After approximately 3 h, the reaction was in equilibrium and 270 g/h of
liquid product containing 51 g of MPC and 10.5 g of DPC were taken off via
the siphon. This corresponds to a space-time yield of 66.6 g/1 h. The
selectivity here was also 99.9%.
Example 4
Example 2 was repeated with the reaction conditions and starting material
streams specified there. In addition, continuous introduction of the
liquid phase taken off at reactor A (FIG. 3) was carried out at the upper
end of an additional bubble column reactor (reactor D in FIGS. 1 and 2).
This bubble column reactor (of identical type to reactor A) was likewise
provided with jacket heating (thermostatted with oil to 180.degree. C.).
Simultaneously with the liquid phase, a nitrogen stream of 100 1 (S.T.P.)/h
was preheated in an electrically heated tube and metered in at the lower
end of the additional bubble column. After 6 h, the reaction was in
equilibrium.
At the lower end of the second bubble column, 767.3 g/h of liquid product
mixture containing 21.1 g of MPC, 85.4 g of DPC and 660.8 g of phenol
continuously ran off via an outlet. In a freezer trap, 25 g of a mixture
of DMC and methanol condensed out of the nitrogen stream per hour. This
corresponds to a space-time yield for MPC and DPC of 56.1 g/1 h, based on
the total reaction volume of the two reactors.
Comparative Example
A heated stirred vessel having 11 internal volume, which was equipped with
a 1 m long column filled with 4.times.4 mm glass rings was filled with 500
g of phenol and 11 g of titanium tetraphenolate. After heating up the
vessel contents to 175.degree. C. to 180.degree. C., the metering in of
the DMC was performed in such a way that the internal temperature did not
decrease. In the course of 4 h, 78 g of DMC were metered in. At the same
time, 49.1 g of a mixture of DMC and methanol distilled off via the
column. The bottom product after this time was composed of 451.4 g of
phenol, 58.5 g of MPC, 13 g of DPC, 2.2 g of by-products and 3.7 g of DMC.
From this there results a phenol conversion rate of 9.7% and a selectivity
of 97.9%, based on converted phenol. The space-time yield for the
formation of the aromatic carbonates was thus 8.94 g/1 h.
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